8 MICROSOMAL PARTICLES 



per gram of particle was detected, but it is at most a few per cent of that found 

 in equal weights of liver microsome fractions. 



DISCUSSION 



The study of A. vinelandii ribonucleoprotein has been more successful in 

 posing interesting questions than in providing answers to previous questions. 

 Can the sharp requirements of the nucleoprotein for divalent cations and for 

 certain pW ranges be exploited to give information on the mode of combina- 

 tion of nucleic acid and protein or combination between nucleoprotein subunits ? 

 Does the marked difference in ribonuclease sensitivity of the plant ribonucleo- 

 protein viruses and the bacterial ribonucleoprotein imply a different orientation 

 or localization of nucleic acid and protein? The plant viruses now appear to 

 have a protein coating with nucleic acid (or nucleoprotein) in an inner layer 

 concentric with the protein coat and thus protected from ribonuclease attack. 

 How different must the structure be to permit the rapid attack observed? 



Will the small subunits derivable by salt and EDTA treatment also yield in- 

 formation on the mode of action or size of the functional nucleic acid? 



The striking effect of divalent cations on the physical state of nucleoprotein 

 is now becoming recognized as a phenomenon common to many systems. 

 Huiskamp [2] in 1901 noted that thymus nucleoprotein was precipitated by 

 0.01 M calcium, barium, and magnesium salts and dissolved in excesses (0.1 M) 

 of these same salts. He equated changes in physical properties of nucleoprotein 

 solutions during dialysis to losses of divalent cations. He noted that heavy- 

 metal divalent cations formed nucleoprotein precipitates that were difficult to 

 dissolve. Korkes [22] and co-workers used a similar observation on manganese 

 RNP to remove RNP from bacterial extracts. Carter and Hall [23] working 

 in the laboratories of J. W. Williams noted that thymus nucleoprotein in 

 sodium chloride solutions was a rodlike molecule but in calcium chloride solu- 

 tions it became compact and showed no dependence of sedimentation rate on 

 concentration. Wiberg and Neuman [24] have studied the binding of mag- 

 nesium and calcium by RNA and DNA and find a region of concentration 

 through which the number of equivalents bound changes rapidly. This con- 

 centration range is the same as that which we find critical for nucleoprotein 

 structural changes. 



The studies reported here, added to the work of Mazia [25] on the role of 

 polyvalent cations in deoxynucleoprotein and nuclear structure and of Chao 

 and Schachman [14a] in ribonucleoprotein stability, and to the many excellent 

 contributions presented at the second annual Biophysics Conference, will help 

 establish the basic rules for fractionation of subcellular particles in a reproducible 

 manner. 



The dependence of nucleoprotein structure upon divalent cation concentra- 

 tion is striking enough for these ions to become of interest in consideration of 

 the variables which dictate when a nucleic acid will be in the double helix, 



